Peter Smithson wrote:

Anyway, now they've just got some people saying it's OK to put a flue
through two walls and another person saying its against regulation to
put it through two walls! Might try and find these regs on the
internet.

I think it unlikely that any manufacturers will forbid a modern fanned
flue going through two walls.
They do specify maximum equivalent [1] lengths of flue which you will
have to comply with.
In fact given the price structure of the flue extending parts they'll
likely be quite pleased with a flue through two walls.


[1] The length allowing for bends usually 1m per Elbow.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html